Patients with acute leukemia and other advanced malignancies most frequently develop infection caused by organisms with which they are colonized. The purpose of this project is to study the patients' microbial flora by means of surveillance cultures or surveys and to monitor personnel for the presence of harmful microorganisms. Clinically important bacterial strains are subspeciated by means of phage typing, serotyping, or biotyping. A quantitative evaluation of the aerobic and anaerobic rectal flora of patients who received gentamicin and nystatin to suppress total enteric flora vs. trimetheprim-sulfamethoxazole plus nystatin to suppress only facultative organisms showed that the latter regimen suppresses facultative bacteria, but leaves the anaerobic organisms present to provide colonization resistance. A new medium has been devised to survey patients for an antibiotic resistant coryneform bacterium. Other surveys were conducted to detect fungi in nasal cultures, S. aureus in personnel and the source of an S. mitis bacteremia.